PROJECT SUMMARY/ABSTRACT Childhood obesity is a critical public health threat, and a major risk factor for accelerating cardiometabolic health conditions in childhood and adulthood. In addition, obesity in childhood negatively impacts academic achievement, which predicts lifetime health outcomes and has dramatic economic implications. Daycare settings have been identified as a high impact target for early childhood obesity intervention. On average, 75% of preschool-aged children in the United States spend 35 hours per week in daycare, and childcare settings have the ability to offer children healthy meals, increase opportunities for physical activity, and promote adequate sleep ? all of which help to prevent obesity. Although preschool settings offer regular opportunities for physical activity, most children in daycare do not meet current guidelines. In 2014, only 12% of daycare centers in Washington State met goals for physical activity, and only 22% for outdoor time. Because children who spend more time outdoors are more physically active than those who spend more time indoors, preschools with outdoor- and play-based curricula may help to increase physical activity. The only published study on this topic found that Swedish children in outdoor preschools were less likely to be overweight and slept longer than their peers in indoor preschools. However, the study was cross-sectional, recruited a small sample, did not examine academic achievement or cost implications, and was conducted in a country with different healthcare and educational systems than the United States. For this K01 application, we propose a prospective observational study that leverages a natural experiment to evaluate the impact of an outdoor preschool model on health outcomes and academic achievement. We will partner with Tiny Trees, a preschool in Seattle, Washington, with an entirely outdoor, play-based curriculum. We will recruit 100 children ages 3-5 who attend Tiny Trees, and 100 children from the Tiny Trees waitlist who are currently attending a traditional indoor preschool. We will collect objective and subjective measures of physical activity, sleep, body mass index, and academic performance over the course of the 3-year study period. We will also perform a cost-benefit analysis to evaluate the Tiny Trees outdoor preschool model for longer-term sustainability. Our Specific Aims are to: 1) compare within-person change in health and academic outcomes between Tiny Trees enrollees (intervention) and waitlisted children (control), 2) estimate the short- and long-term cost-benefit of the Tiny Trees outdoor preschool model, and 3) complete a rigorous mentored career development plan that will position the Principal Investigator to conduct groundbreaking research in interventions to prevent childhood obesity, establish cost-benefit estimates of interventions to inform policy, and launch a successful career as an independent investigator. This proposal aligns with the National Heart, Lung, and Blood Institute agenda by prioritizing novel epidemiological research to prevent childhood obesity, a critical risk factor for cardiovascular disease, and will yield much-needed cost- benefit data to examine the capacity for obesity prevention and sustainability of an outdoor preschool model.